Butterfly bushes (Buddleia) do not require pruning. However, a once a year hard pruning and regular deadheadng throughout the season can be very beneficial.
Deadheading During the Active Growth Season
There is a technique known as "dead-heading", it is a simple task which takes a few minutes however adds days and sometimes weeks to your flowering display. And no, this has nothing to do with following a rock band around on their nationwide tour. :-)
How to Dead-head
If you've never dead-headed before here's how to go about it: First, keep a watchful eye on your flowering plants, paying close attention to blooms that are past their best. Once a flower has started to fade remove it from the plant with a quick snip from your pruners . When doing this try to remove just the spent flower leaving the new buds beneath intact.
And that's about all there is to dead-heading!
Hard Pruning in Winter
Many varieties of Butterfly Bush can become quite large (up to 12' tall). You can hard-prune your Butterfly Bush in late Winter, just before new growth typically emerges. Butterfly Bush should be pruned back by 50% or more. You can actually prune them to stumps just 4 to 6 inches above the ground.
In northern zones: In regions that receive alot of snow and severe cold temperatures during the winter months, it is highly recommended to wait until Spring arrives to prune Butterfly Bush. Leave the canes to trap and hold a layer of insulating snow around the base of the plant throughout the coldest parts of the winter. Then, when snow has left for the season, prune.
